Adsorption processes are widely used in industry for separation of fluid mixtures (gas or liquid). The separation is based on preferential adsorption of selective components on the surface of solid adsorbents. For efficient separation, the adsorbent material must have large surface areas to provide reasonable adsorptive capacities. The commonly used adsorbents, such as molecular sieve zeolites, activated carbon, alumina and silica gel, have surface areas of at least 200 m.sup.2 /g.
Most industrial adsorption processes are carried out in fixed-bed type columns. The adsorbent granules are packed and immobilized in a cylindrical vessel. As the fluid mixture to be separated is passed through the packing via the void spaces among the granules, the adsorbable components in the mixture are taken up and retained by the adsorbent.
Since the adsorbent has a limited adsorption capacity, it will become gradually saturated with adsorbate, and periodic adsorbent regeneration is required. For continuous processing of a feed mixture, a multi-bed system is used in which each bed goes through the adsorption/regeneration cycle in sequence. Several different regeneration methods have been used commercially. Chief among them are the thermal swing adsorption (TSA) and pressure swing adsorption (PSA) processes. In the TSA process, the saturated adsorbent is regenerated by purging with a hot gas. Each heating/cooling cycle usually requires a few hours to over a day. In the PSA process, the adsorbent regeneration is effected by purging with a portion of the purified product gas at reduced pressure. The throughput is higher than that of the TSA since faster cycles, usually in minutes, are possible.
Apart from the adsorptive capacity of the adsorbent, the adsorption rate and pressure drop are two important factors that must be considered in adsorber design.
Pressure drop through the adsorber column should be minimized, because high fluid pressure drop can cause movement or fluidization of the adsorbent particles, resulting in serious attrition and loss of the adsorbent.
The adsorption rate has a significant bearing on the efficiency of the adsorption process. This rate is usually determined by the mass transfer resistance to adsorbate transport from the bulk fluid phase to the internal surfaces of the adsorbent particles. Slow adsorption rate due to large mass transfer resistance will result in a long mass transfer zone (MTZ) within which the adsorbent is only partially saturated with adsorbate. The adsorbent in the region upstream of the MTZ is substantially saturated with adsorbate, while that downstream of the MTZ is essentially free of adsorbate. As the fluid continues to flow, the MTZ advances through the adsorber column in the direction of the fluid stream. The adsorption step must be terminated before the MTZ reaches the adsorber outlet in order to avoid the breakthrough of adsorbate in the effluent stream. A long mass transfer zone, which contains a large quantity of partially utilized adsorbent, will, therefore, result in a short adsorption step and inefficient use of the adsorbent capacity. These effects are especially serious for the pressure swing adsorption process.
Both the pressure drop and the mass transfer resistance are strongly influenced by the size of the adsorbent particles. Changing the particle size, unfortunately, has opposite effects on these two important factors. This is elaborated below:
(1) The pore sizes of the void spaces among the adsorbent particles in the fixed-bed are proportional to the size of the particles. Since the resistance to the fluid flow through the adsorber is inversely proportional to the pore size of the packed bed, the use of small adsorbent particles will cause high pressure drop. For this reason, the sizes of particles of commercial adsorbents for fixed-bed operation are generally larger than 2 mm in equivalent diameter. Adsorbent of smaller particle sizes, such as zeolite crystals (less than 10 microns), are pelletized using binding material to suitable sizes.
(2) Almost all the surface areas of commercial adsorbents are located at the interior of the adsorbent particle. For adsorption to occur, the adsorbate needs to be transported from the external fluid phase to the interior surface of the particle. The transport rate is dominated by two mass transfer mechanisms in series: (a) interfacial mass transfer--diffusion through the fluid boundary layer surrounding the external surface of the adsorbent particle; and (b) intraparticle mass transfer--diffusion through the internal pore space (micropores and macropores) of the particle to its interior surface where adsorption takes place. The size of the particle has significant effects on the rates of these two diffusion processes. Small particles offer large fluid/solid contact areas in the fixed bed for interfacial mass transfer and reduce the path length for the intraparticle diffusion. Hence, small adsorbent particles will increase adsorption rate and result in a narrow mass transfer zone for fast and efficient operation of adsorption/desorption cycles.
The above discussions and analysis show that small adsorbent particles are desirable for efficient adsorption processes, but the minimum particle size is limited by acceptable hydrodynamic operating conditions of the fixed bed adsorber. That is, one wants to avoid fluidization and excessive pressure drop. Such a concept also applies to a heterogeneous catalytic reaction process, which involves an adsorption step in the reaction mechanism. The use of small catalyst particles will enhance mass transfer between the catalyst and surrounding fluid carrying the reactants, but it will also increase pressure drop through the reactor bed.
It would therefore be desirable to provide an adsorber or catalytic reactor containing adsorbent or catalyst characterized by a relatively small particle size and yet still able to operate with an acceptable pressure drop.
At this point, it is appropriate to shortly describe the structure and operation of a known separation device used for permeation and absorption and referred to as a hollow fiber module. As will become clear below, this module is similar in many respects to a shell and tube heat exchanger. The device is used to separate at least one component (e.g. CO.sub.2) from a second `carrier` component (e.g. natural gas) with which it forms a feed mixture. A typical module comprises a cylindrical vessel encapsulating a bundle of small-diameter, elongated, hollow fibers. The fibers are formed of a material having a permeability which, in the case of a permeation module, is selected to allow the component to be extracted to diffuse therethrough but to substantially reject the carrier component. In the case of an absorption module, the entire feed mixture may readily diffuse through the fiber wall. The fibers are "potted" at their ends in closure means, such as epoxy tube sheets, so that the ends of the fibers project therethrough, leaving their bores or "lumina" open. The tube sheets function to seal the void space between the fibers at the two ends. The tube sheets further seal or are sealed by means, such as an O-ring, against the inside surface of the vessel. The vessel is provided with a first inlet and first outlet communicating with the ends of the fiber lumina. It further has a second inlet and second outlet communicating with the ends of the void space. In operation, the feed mixture of gases is fed through the second inlet into the void space. In the case of an absorption module, absorbent fluid is fed into the lumina. The absorbate (CO.sub.2) diffuses through the fiber walls from the void space, is collected by the absorbent fluid, and exits through the first outlet. The carrier gas, reduced in CO.sub.2, leaves through the second outlet.
With this background in mind, it is now appropriate to describe the present invention.